Purification system for recovering monomers and raw materials in polyacrylonitrile preparation

Abstract

The utility model discloses a purification system for recovering monomers and raw materials in polyacrylonitrile preparation. The purification system comprises a polymerization system, a steam stripping system, a monomer purification system and a preparation system, unreacted monomers and water in the polyacrylonitrile slurry are evaporated out from the tower top by the stripping tower and flow into a monomer purification system; the monomer purification system comprises a decantation tank and a rectifying tower, unreacted monomers and water flowing out of the stripping tower flow into the decantation tank and are preliminarily separated into a monomer aqueous solution and a recycled monomer floating on the upper layer of the monomer aqueous solution under the action of gravity in the decantation tank; the upper-layer recycled monomer flows into the rectifying tower from the upper feeding hole to be rectified and purified to obtain a purified monomer. The device can be used for simultaneously purifying two different recovered monomers with large impurity content difference and fresh acrylonitrile, and can be used for flexibly adjusting the position of the feeding hole of the rectifying tower according to the adjustment of the impurity content in the recovered monomers, so that the recovered monomers with high impurity content can be stably separated.

Description

Technical Field

[0001] The utility model relates to a purification system for recovering monomers and raw materials in the preparation of polyacrylonitrile, and belongs to the technical field of preparation of polyacrylonitrile. Background Art

[0002] The polymerization processes used in the preparation of polyacrylonitrile-based carbon fibers primarily include homogeneous solution polymerization (one-step) and heterogeneous solution polymerization (two-step). In the one-step method, acrylonitrile is directly polymerized in a solvent such as DMSO (dimethyl sulfoxide). The resulting slurry is then subjected to a stripping tower to remove monomers, resulting in a spinning solution that can be directly used for spinning. The recovered monomers do not undergo further purification or polymerization. In the two-step aqueous suspension polymerization method, acrylonitrile and other monomers, water, and reaction aids undergo polymerization to produce a polyacrylonitrile slurry. This slurry undergoes stripping, filtration, and drying to produce polyacrylonitrile powder, which is then prepared into a spinning solution for spinning through a stock solution step. In the stripping step, unreacted monomer is condensed and recovered at the top of the stripping tower. The aqueous and monomer phases separate in a decanter, and the recovered monomer enters a monomer recovery tank. The recovered monomer is quantitatively added to a mixed monomer tank to prepare the mixed monomer with fresh acrylonitrile and other monomers. The amount of recovered monomer added depends on the polymerization conversion rate.

[0003] In addition to acrylonitrile, the recycled monomers also contain impurities such as water, acetaldehyde, acetone, methyl paraben (MQ), oxazole, and metal ions. Fluctuating levels of iron ions adversely affect the polymerization rate, conversion rate, polymer particle size, molecular weight, and PD value using ferrous sulfate-ammonium bisulfite as a redox system. This monomer removal system requires the addition of sufficient MQ before the decanting tank to prevent monomer self-polymerization. Inhibitors cause an induction period (a period of zero polymerization rate) during polymerization, the length of which is proportional to the inhibitor content. Excessive inhibitors can affect polymer molecular weight and, at high temperatures, produce dark matter, ultimately impacting product quality. Acrolein, acetone, and propionitrile contain highly electronegative polar groups in their molecular structures, making the hydrogen atoms attached to them highly reactive and contributing to chain transfer. These impurities can enter the polymer during acrylonitrile polymerization, affecting the regularity of the macromolecule and significantly impacting subsequent spinning and carbonization.

[0004] The recovered monomer is mixed with fresh acrylonitrile to prepare a mixed monomer. After dilution, the acetone, acetonitrile, propionitrile, butenenitrile, MQ, oxazole, acrolein and metal ions in the mixed monomer are still high, and some impurities exceed the national standard upper limit. Therefore, it is necessary to purify the mixed monomer.

[0005] Patent CN115671771A proposes that after the acrylonitrile raw material and sodium hydroxide solution are uniformly mixed in a distillation kettle with a stirrer, the impurities in the acrylonitrile raw material are removed by distillation in a distillation tower by means of negative pressure distillation. Total aldehyde, total cyanide, acrolein, acetone, propionitrile, crotononitrile, MQ, peroxide, copper ions, and iron ions are not detected. Oxazole, acetonitrile, and methacrylonitrile cannot be completely removed because their boiling points are similar to those of acrylonitrile.

[0006] This patent introduces a new impurity, sodium hydroxide, before distillation, causing the pH of the acrylonitrile raw material to change. The aqueous suspension polymerization must be carried out under acidic conditions (pH < 3.1) to ensure the activity of the activator and initiator and the generation of free radicals in the redox system.

[0007] Patent CN1021199105B provides a method for preparing high-purity acrylonitrile, which does not introduce new compounds and new impurities and simultaneously removes multiple impurities from the raw material acrylonitrile. A cation exchange resin is used to remove oxazole, an anion exchange resin to remove acrolein, and aluminum oxide to remove peroxide and water. Finally, a high-purity acrylonitrile with a purity of up to 99.99% is obtained by extracting it through a side line of a rectifying tower, removing acetaldehyde, acetone, acrolein, methacrylonitrile, oxazole, crotononitrile, inhibitor, hydrocyanic acid, and iron ion. This set of processes is applicable to the purification of high-quality acrylonitrile industrial products. It has a relatively high purification capacity for mixed monomers with a relatively high impurity concentration and is related to three sets of adsorption devices. The residence time of acrylonitrile in the adsorption tower is too long, and water in the aqueous suspension polymerization is used as a carrier and does not need to be removed.

[0008] In summary, the existing technology has obvious inconveniences and defects in actual use, so it is necessary to improve it. Utility Model Content

[0009] The utility model addresses the deficiencies in the background technology and provides a system for purifying recovered monomers and raw materials in the preparation of polyacrylonitrile. The system can simultaneously purify two recovered monomers with greatly different impurity contents and fresh acrylonitrile. The position of the feed port of a distillation tower can also be flexibly adjusted according to the adjustment of the impurity content in the recovered monomers to ensure that the recovered monomers with high impurity content can be stably separated. Anion and cation exchange resins are used to remove oxazole and acrolein. A pure mixed monomer is finally obtained through the three steps of preliminary separation, distillation purification, and deep purification, thereby reducing the occurrence of side reactions in the polymerization reaction, improving the stability of the polymerization reaction, and providing high-quality polyacrylonitrile powder with uniform molecular weight and few residual impurities for downstream stock solution preparation.

[0010] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0011] A purification system for recovering monomers and raw materials in the preparation of polyacrylonitrile, comprising a polymerization system, a stripping system, a monomer purification system and a preparation system;

[0012] The stripping system includes a stripping tower. The polyacrylonitrile slurry flows from the polymerization system into the stripping tower. The stripping tower evaporates the unreacted monomers and water in the polyacrylonitrile slurry from the top of the tower and flows into the monomer purification system.

[0013] The monomer purification system includes a decanting tank and a distillation tower. Unreacted monomers and water flowing out of the stripping tower flow into the decanting tank and are initially separated into a monomer aqueous solution and a recovered monomer floating on the upper layer of the monomer aqueous solution under the action of gravity in the decanting tank.

[0014] The recovered monomers in the upper layer flow into the distillation tower from the upper feed port for distillation and purification to obtain purified monomers;

[0015] The monomer purification system further includes a first adsorption tower and a second adsorption tower. The purified monomer flows out from the discharge port of the distillation tower and flows through the first adsorption tower and the second adsorption tower in sequence.

[0016] Furthermore, the first adsorption tower is provided with a cationic resin for removing oxazole in the purified monomer, and the second adsorption tower is provided with an anionic resin for removing acrolein in the purified monomer, and finally the acrylonitrile is deeply purified into high-purity acrylonitrile.

[0017] Furthermore, the polymerization system includes a polymerization kettle, an overflow tank and a slurry pump. The polyacrylonitrile slurry generated by the polymerization kettle overflows the overflow tank and is then transported to the stripping tower by the slurry pump.

[0018] Furthermore, the stripping system further comprises a primary cooler and a secondary cooler, and the unreacted monomers and water evaporated from the stripping tower are condensed into a liquid phase through the primary cooler and the secondary cooler.

[0019] Furthermore, the monomer aqueous solution in the decantation tank is transported to the middle tray of the stripping tower by a shielded pump for secondary stripping to further recover acrylonitrile therein.

[0020] Furthermore, fresh acrylonitrile transported from the raw material tank area enters the lower feed port of the distillation tower, so that the recovered monomer and fresh acrylonitrile are fed in through double side lines.

[0021] Furthermore, the position of the upper feed port on the distillation tower is adjustable up and down. When the impurity content of the light component in the recovered monomer is higher, the upper feed port is positioned closer to the top of the distillation tower.

[0022] Furthermore, the monomer purification system also includes a condenser and a reflux tank arranged at the top of the distillation tower. The vacuum pump evacuates the condenser, and the acrylonitrile containing light component impurities at the top of the distillation tower is condensed in the condenser and then flows to the reflux tank for collection.

[0023] Furthermore, the preparation system includes a mixed monomer tank, into which deeply purified high-purity acrylonitrile flows, and other monomers are added for mixing according to process requirements. The acrylonitrile finally enters the polymerization kettle through the pumping action of the monomer feed pump to participate in the next round of polymerization reaction.

[0024] Compared with the prior art, the present invention has the following advantages after adopting the above technical solution:

[0025] The polyacrylonitrile slurry is initially separated by gravity into a monomer aqueous solution and a recovered monomer floating on the upper layer of the monomer aqueous solution in a decantation tank. The recovered monomer in the upper layer flows into a distillation tower from an upper feed port for distillation and purification to obtain a purified monomer, which then flows through the first adsorption tower and the second adsorption tower in sequence and is finally deeply purified into high-purity acrylonitrile. The purification process is safe and reliable, and can effectively remove various impurities in acrylonitrile, avoiding problems such as decreased polymerization reaction conversion rate and degree of polymerization, low molecular weight of polyacrylonitrile powder, and high PD value caused by excessively high impurity content in the mixed monomer, thereby providing high-quality polyacrylonitrile powder for the production of high-performance precursor and carbon filaments.

[0026] The present invention will be described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the process of the present utility model.

[0028] In the figure, 1-polymerization kettle, 2-overflow tank, 3-slurry pump, 4-stripping tower, 5-primary cooler, 6-secondary cooler, 7-decantation tank, 8-distillation tower, 801-upper feed port, 802-lower feed port, 803-discharge port, 9-first adsorption tower, 10-second adsorption tower, 11-mixed monomer tank, 12-monomer feed pump, 13-reflux pump, 14-reflux tank, 15-condenser, 16-vacuum pump, 17-shielded pump. DETAILED DESCRIPTION

[0029] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the specific implementation methods of the present invention are now described with reference to the accompanying drawings.

[0030] like Figure 1 As shown, the utility model provides a purification system for recovering monomers and raw materials in the preparation of polyacrylonitrile, including a polymerization system, a stripping system, a monomer purification system and a preparation system;

[0031] The stripping system includes a stripping tower 4. The polyacrylonitrile slurry flows from the polymerization system into the stripping tower 4. The stripping tower 4 evaporates the unreacted monomers and water in the polyacrylonitrile slurry from the top of the tower and flows into the monomer purification system.

[0032] The monomer purification system includes a decanter 7 and a distillation tower 8. The unreacted monomer and water flowing out of the stripping tower 4 flow into the decanter 7 and are initially separated into a monomer aqueous solution (containing water-soluble impurities and a small amount of acrylonitrile) and a recovered monomer (acrylonitrile containing impurities) floating on the upper layer of the monomer aqueous solution under the action of gravity.

[0033] The recovered monomers in the upper layer flow into the distillation tower 8 from the upper feed port 801 for distillation and purification to obtain purified monomers;

[0034] The monomer purification system also includes a first adsorption tower 9 and a second adsorption tower 10. The purified monomer flows out of the discharge port 803 of the distillation tower 8 and flows through the first adsorption tower 9 and the second adsorption tower 10 in sequence. The first adsorption tower 9 is equipped with a cationic resin to remove oxazole from the purified monomer, and the second adsorption tower 10 is equipped with an anionic resin to remove acrolein from the purified monomer, thereby deeply purifying the monomer into high-purity acrylonitrile. The cation exchange resin in the first adsorption tower 9 uses activated D001 macroporous styrene as a strong acid cation exchange resin, with a material space velocity (V / s) of 200 h⁻¹ and a processing temperature of 15-20°C. The anion exchange resin in the second adsorption tower 10 uses activated 201 gel-type styrene as a strong base anion exchange resin, with a material space velocity (V / s) of 200 h⁻¹ and a processing temperature of 15-20°C.

[0035] Oxazole, acetonitrile, and methacrylonitrile have similar boiling points to acrylonitrile, making distillation ineffective. Over time, oxazole and acrolein accumulate in the recovered monomers. Oxazole, being weakly basic, undergoes a nucleophilic reaction with the acidic groups on the cation exchange resin, resulting in adsorption onto the resin and separation from acrylonitrile. Trace amounts of acrolein are a key factor in carbon fiber defects. The amine groups on the anion exchange resin condense with acrolein, adsorbing it onto the resin. Although acetonitrile and methacrylonitrile have similar structures to acrylonitrile, their low content in the monomer mixture reduces their impact on the polymerization reaction. D001 macroporous styrene-based strong acid cation exchange resin was selected for the cation exchange resin, while D201 gel-type macroporous strong base anion exchange resin was selected for the anion exchange resin. Their macroporous structure effectively accommodates and exchanges large molecules or high-concentration ions, reducing the flow resistance of the solution through the resin and preventing the self-polymerization of acrylonitrile that can be caused by the small pore structure. The first adsorption tower 9 and the second adsorption tower 10 are used in series. The resin must be pretreated and activated before use, and regenerated after a period of use to restore its exchange capacity.

[0036] The polymerization system includes a polymerization kettle 1, an overflow tank 2, and a slurry pump 3. In the polymerization kettle 1, an aqueous suspension polymerization process is used. Free radicals generated by the redox reaction of the polymerization kettle initiate polymerization of the monomers to produce a polyacrylonitrile slurry. The redox system adopts a persulfate-bisulfite-ferrous sulfate redox system. The activity, number, and iron content of the free radicals in the polymerization kettle directly affect the polymerization reaction. The polyacrylonitrile slurry generated in the polymerization kettle 1 overflows the overflow tank 2 and is then transported by the slurry pump 3 to the stripping tower 4. The polyacrylonitrile slurry enters the stripping tower 4 from the top and contacts the low-pressure steam entering the tower from the bottom on the tower plates in countercurrent. The temperature at the top of the stripping tower 4 is controlled at 93-96°C.

[0037] The stripping system further includes a primary cooler 5 and a secondary cooler 6. Unreacted monomers and water evaporated from the stripping tower 4 are condensed into a liquid phase through the primary cooler 5 and the secondary cooler 6. Sufficient hydroxyanisole is added between the primary cooler 5 and the secondary cooler 6 to prevent monomer self-polymerization.

[0038] The monomer aqueous solution in the decantation tank 7 is transported by the shielded pump 17 to the middle tray of the stripping tower 4 for secondary stripping to further recover the acrylonitrile therein.

[0039] Fresh acrylonitrile transported from the raw material tank area enters the lower feed port 802 of the distillation tower 8, so that the recovered monomer and fresh acrylonitrile are fed in through double side lines.

[0040] Furthermore, the position of upper feed port 801 on distillation tower 8 is adjustable. When the impurity content of light components in the recovered monomer is higher, the position is closer to the tower top, ensuring stable separation of the high-proportion light impurities. In the early stages of the plant's operation, when the impurity content of the recovered monomer is low, it is mixed with fresh acrylonitrile through a bypass pipe in a static mixer before entering the distillation tower 8 through lower feed port 802. The distillation action of distillation tower 8 not only removes light impurities such as acetone, propionitrile, acetaldehyde, and crotononitrile, but also effectively removes heavy impurities such as p-hydroxyanisole (MQ) and metal ions (Fe, Cu). The position of upper feed port 801 can be flexibly adjusted. When the cumulative impurity content exceeds the specified limit, dual side-line feeding is switched to fully utilize the tower space and mass transfer area, maximizing separation efficiency. Furthermore, the location and number of side-line extractions can be rationally adjusted based on the temperature distribution and heat demand within the tower, further improving heat exchange efficiency and ensuring the separation and purity of each component.

[0041] The monomer purification system also includes a condenser 15 and a reflux tank 14 disposed at the top of the distillation tower 8. A vacuum pump 16 evacuates the condenser 15. Acrylonitrile containing light component impurities at the top of the distillation tower 8 is condensed by the condenser 15 and then flows to the reflux tank 14 for collection. The acrylonitrile is then transported into the tower by the reflux pump 13 at a certain reflux ratio. The light component impurities are extracted and collected; the heavy component impurities are extracted and collected at the bottom of the distillation tower 8.

[0042] The preparation system includes a monomer mixing tank 11, into which highly purified, high-purity acrylonitrile flows, along with other monomers added for mixing as required by the process. The acrylonitrile is then pumped by a monomer feed pump 12 and ultimately enters the polymerization reactor 1 to participate in the next round of polymerization. The purification process of this utility model is safe and reliable, effectively removing various impurities from acrylonitrile. This prevents problems such as decreased polymerization conversion rate and degree of polymerization, low molecular weight of polyacrylonitrile powder, and high PD values caused by excessive impurity content in the mixed monomers, thereby providing high-quality polyacrylonitrile powder for the production of high-performance precursor and carbon filaments.

[0043] The above description is merely an example of the preferred embodiment of the present invention. Any details not described in detail are common knowledge within the art. The scope of protection of the present invention is determined by the claims. Any equivalent modifications based on the technical teachings of the present invention are also within the scope of protection of the present invention.

Claims

1. A purification system for recovering monomers and raw materials in the preparation of polyacrylonitrile, characterized by: Including polymerization system, stripping system, monomer purification system and preparation system; The stripping system includes a stripping tower (4), into which polyacrylonitrile slurry flows from the polymerization system. The stripping tower (4) evaporates unreacted monomers and water in the polyacrylonitrile slurry from the top of the tower and flows into the monomer purification system. The monomer purification system comprises a decanting tank (7) and a distillation tower (8); unreacted monomers and water flowing out of the stripping tower (4) flow into the decanting tank (7) and are initially separated into a monomer aqueous solution and a recovered monomer floating on the upper layer of the monomer aqueous solution under the action of gravity in the decanting tank (7); The recovered monomers in the upper layer flow from the upper feed port (801) into the distillation tower (8) for distillation and purification to obtain purified monomers; The monomer purification system further comprises a first adsorption tower (9) and a second adsorption tower (10); the purified monomer flows out from the discharge port (803) of the distillation tower (8) and flows through the first adsorption tower (9) and the second adsorption tower (10) in sequence.

2. The purification system for recovering monomers and raw materials in the preparation of polyacrylonitrile according to claim 1, characterized in that: The first adsorption tower (9) is provided with a cationic resin for removing oxazole in the purified monomer, and the second adsorption tower (10) is provided with an anionic resin for removing acrolein in the purified monomer, and finally the acrylonitrile is deeply purified into high-purity acrylonitrile.

3. The purification system for recovering monomers and raw materials in the preparation of polyacrylonitrile according to claim 1, characterized in that: The polymerization system comprises a polymerization kettle (1), an overflow tank (2) and a slurry pump (3). The polyacrylonitrile slurry generated in the polymerization kettle (1) overflows the overflow tank (2) and is then transported to a stripping tower (4) by the slurry pump (3).

4. The purification system for recovering monomers and raw materials in the preparation of polyacrylonitrile according to claim 1, characterized in that: The stripping system further comprises a primary cooler (5) and a secondary cooler (6), and the unreacted monomers and water evaporated from the stripping tower (4) are condensed into a liquid phase through the primary cooler (5) and the secondary cooler (6).

5. The purification system for recovering monomers and raw materials in the preparation of polyacrylonitrile according to claim 1, characterized in that: The monomer aqueous solution in the decantation tank (7) is transported by a shielded pump (17) to the middle tray of the stripping tower (4) for secondary stripping to further recover the acrylonitrile therein.

6. The purification system for recovering monomers and raw materials in the preparation of polyacrylonitrile according to claim 1, characterized in that: Fresh acrylonitrile transported from the raw material tank area enters the lower feed port (802) of the distillation tower (8), so that the recovered monomer and fresh acrylonitrile are fed in through a double side line.

7. The purification system for recovering monomers and raw materials in the preparation of polyacrylonitrile according to claim 6, characterized in that: The position of the upper feed port (801) on the distillation tower (8) is adjustable up and down. When the impurity content of the light component in the recovered monomer is higher, the upper feed port (801) is closer to the top of the distillation tower (8).

8. The purification system for recovering monomers and raw materials in the preparation of polyacrylonitrile according to claim 1, characterized in that: The monomer purification system further comprises a condenser (15) and a reflux tank (14) arranged at the top of the distillation tower (8). A vacuum pump (16) evacuates the condenser (15). Acrylonitrile containing light component impurities at the top of the distillation tower (8) is condensed by the condenser (15) and then flows to the reflux tank (14) for collection.

9. The purification system for recovering monomers and raw materials in the preparation of polyacrylonitrile according to claim 1, characterized in that: The preparation system includes a monomer mixing tank (11), into which highly purified high-purity acrylonitrile flows, and other monomers are added for mixing according to process requirements. The acrylonitrile finally enters the polymerization kettle (1) through the pumping action of a monomer feed pump (12) to participate in the next round of polymerization reaction.

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